A growing number of double-blind placebo-controlled studies have considered the influence of micro-nutrient supplementation on the intelligence of children. Earlier studies prevented the drawing of conclusions as they did not systematically approach the topic. However, over the last 10 years, a series of studies have compared the impact of supplementation on either verbal or non-verbal measures of intelligence. In 10 out of 13 studies a positive response has been reported, always with non-verbal measures, in at least a sub-section of the experimental sample. A selective response to non-verbal tests was predicted as they reflect basic biologically functioning that could be expected to be influenced by diet. The evidence is that not all children respond to supplementation, rather there is a minority who benefit, whose diet offers low amounts of micro-nutrients. Such observations are consistent with dietary surveys that typically report a sub-set of children with a low intake. The topic is at a very early stage and needs the clarification gained from a series of large-scale studies that consider children of a wide range of ages, dietary styles and social backgrounds.